This invention relates to an apparatus for sensing variations in the heat exchange properties of a medium, and more particularly to an apparatus for sensing variations in the flow of a fluid medium such as air or liquid, or for sensing the presence or absence of a liquid as an indication of the level of the liquid in a reservoir.
Various devices have been used in the prior art for sensing variations in the flow of a fluid medium or for sensing liquid level. For sensing fluid flow, for example, various mechanical impact devices have been used. However, they present substantial interference with the fluid flow. Generally, the mechanical devices whether used for sensing fluid flow or for fluid level detection have poor response characteristics and cannot sense relatively small variations in the heat exchange properties of the surrounding medium. They are also susceptible to nuisance operations and subject to wear.
Negative and positive temperature coefficient devices have also been used for sensing variations in the heat exchange properties of a medium. Such devices are normally placed in contact with the medium and the temperature sensed by the devices varies in accordance with the rate at which the medium is drawing heat away from them. Negative and positive temperature coefficient devices can thus be used for measuring fluid flow or for sensing fluid levels. The above prior art arrangements are, however, very sensitive to ambient temperature and power supply variations and means are required for providing adequate compensation. Ambient temperature compensation may be effected by providing a bridge circuit including a second temperature sensitive device which is exposed to ambient temperature but not in contact with the surrounding medium, as disclosed in Canadian Pat. No. 582,848 issued Sept. 8, 1959. However, bridge circuits generally have a very small output which cannot be used directly to control an alarm device and complicated circuitry must be used to amplify the output of the bridge.